The invention is particularly useful as part of document processing, in which individual paper sheets are printed, most often by laser printing, and must be thereafter assembled into bound documents. Typically, an electrostatic laser printer provides a large stack of collated sheets comprising identical or dissimilar multi-page documents, and they must then be separated into their appropriate sets and delivered to a binding machine. For example, each document might be a customized employee benefit statement with internal sections varying in length according to the employee. The problem is to rapidly identify where one document ends and the other begins, and to deliver the lifts in nicely aligned fashion, all at high speed commensurate with the output of modern printers, binding machines and associated equipment.
A rather basic way of providing lifts is to have an operator look manually through the stack for the end of a document and remove the set by hand. More efficiently, high speed machinery has been used to slide the sheets one by one laterally from the stack, to pass them along a conveyor where each sheet is counted or optically inspected, and to deliver sheets to an assembly point where they are jogged into alignment as a lift. In accomplishing such, sheets will be drawn from the end of the stack variously, as by vacuum devices or frictional belts and rollers. See, for example, U.S. Pat. No. 4,928,944 to Golicz. This method is attractive in that each sheet can be scanned as it is removed and the beginning and ends of documents can be identified to form accurate lifts. However, the single sheet separation and assembly method is limited in document production rate because of practical limits of aerodynamics and sheet stiffness on the lateral translation speed of sheets. It is also disadvantageous in that the sliding of sheets by each other creates friction and static electricity; in turn, this may disrupt jogging sheets into re-alignment and may cause electrostatic copier toner transfer.
Other equipment is known which provides lifts without removing single sheets, especially certain devices patented by Pfaffle, as follows. U.S. Pat. No. 3,690,475 shows how a wedge is forced into the side of a vertical stack to define a known length of the stack as a lift which is then pushed out of the stack sideways. In the device of U.S. Pat. No. 3,627,152 a finger and disk enter the corner of a vertical stack and a lift is dropped downwardly onto a conveyor. However, even if the machinery is accurate, slight variations in the thickness or lay of the paper will cause variations in the number of sheets which are removed with each set. U.S. Pat. No. 4,286,908 describes an accessory for compressing a corner to reduce the variations. Thus, defining a thickness of a stack is not a means that can be counted on for precision count. And, it would appear that sheet edges might be hit and damaged from time to time. Certainly, the stack-length measuring method is not particularly adaptable to situations where the documents vary in number of sheets within the stack, or where the thickness of the paper varies within or from one document to the next.
Thus, there is a need for improvement in the method and apparatus by which stacks are separated into lifts. Whatever equipment may be used, it is of course important that minimum operator labor be required to operate the machine. Machines that need constant reloading or adjustment are undesired. Other considerations include simplicity of operation, low maintenance and low capital cost.